st: -heckprob- estimation

Dear All
I'm going to analyze a data using -heckprob-. I have total 79000 respondent
data which were asked a specific question and the responses are 65000
(Yes/No) and 14000 (Don't know). My dependent variable y is thus binary
based on 65000 (Yes/No) and explanatory variables are age, sex, education
and income.
I have created another variable for /select equation / , that is, z=1 if y
(yes/no) and z=0 if y(Don't know). Now when I give the command
heckprob y age sex edu income, select (z=age sex edu income)
it does not concave and iterations are repeated. I read in a book that in
/select equation / , at least one variable should be different. But I don't
know, how I should enter any other variable in /select equation / as I have
the data for only age, sex, education and income.
Any suggestion?
Thanks in advance.
ajjee
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